A slip ring is used in order to ensure a state in which a rotation-side member and a fixed-side member are always kept connected electrically. As the number of poles increases, it becomes necessary to coaxially stack the slip ring in a number corresponding to the number of poles. In a case in which there are few contact points, it is necessary to increase the width of the slip ring and to provide the required contact surface area in order to secure the required transmission capacity. Either approach is detrimental to reducing the thickness of the slip ring because of an increase in the axial length of the ring.
A slip ring suitable for a thickness reduction is disclosed in patent document 1. The slip ring disclosed in the document is configured so that the space between a concentrically arranged moveable-side internal ring and fixed-side external ring is provided with a rotating and revolving planetary gear plate kept in contact with the rings. To ensure that the two rings and the planetary gear plate are kept in contact, an arrangement is adopted in which the rings are elastically held by the planetary gear plate on both sides along the central axis of the rings. A slip ring having this structure can be used to create a multipolar arrangement by placing a plurality of ring pairs composed of internal rings and external rings in a concentric fashion. Increasing the number of planetary gear plates makes it possible to provide the required contact surface area. Accordingly, this arrangement is beneficial to reducing the thickness of the slip ring.
[Patent document 1] Japanese Laid-open Patent Publication No. 5-82223
However, in a slip ring provided with the planetary gear plate of this structure, it is necessary for the internal ring and the external ring to be held on both sides by a planetary gear plate provided with an elastic plate. It is also necessary to place an annular or arcuate retainer between the rings, to attach a shaft to the retainer, and to support the planetary gear plate in a rotatable state by the shaft so that the planetary gear plate can rotate and revolve in the annular space between the two rings. The resulting problem is that the structure becomes more complicated and the number of components increases.